Unconventional magnetism in all-carbon nanofoam

We report production of nanostructured magnetic carbon foam by a high-repetition-rate, high-power laser ablation of glassy carbon in $\mathrm{Ar}$ atmosphere. A combination of characterization techniques revealed that the system contains both $s{p}^2$ and $s{p}^3$ bonded carbon atoms. The material is a form of carbon containing graphite-like sheets with hyperbolic curvature, as proposed for “schwarzite.” The foam exhibits ferromagnetic-like behavior up to $90\mathrm{K}$, with a narrow hysteresis curve and a high saturation magnetization. Such magnetic properties are very unusual for a carbon allotrope. Detailed analysis excludes impurities as the origin of the magnetic signal. We postulate that localized unpaired spins occur because of topological and bonding defects associated with the sheet curvature, and that these spins are stabilized due to the steric protection offered by the convoluted sheets.

Figure 1

Transmission electron micrograph (left), and scanning electron micrograph (right) of the carbon nanofoam, showing the web-like structure. The diffraction pattern in the inset shows the very broad rings indicating the lack of a long-range three-dimensional order in the foam.

Figure 2

High-resolution TEM image of the carbon nanofoam foam (central image) with an individual $6\mathrm{nm}$ cluster clearly seen in the center. Fourier transforms (right) and electron diffraction patterns (left) indicate a “repeat” spacing with characteristic length of $5.6\pm 0.4\mathrm{\AA }$ typical for schwarzites.

Figure 3

Temperature dependence of the mass magnetization of the composite of carbon nanofoam and sample holder (open circles), the carbon nanofoam (filled circles), and the gelatine sample holder (crosses) measured at $30\mathrm{kOe}$ in the temperature range $1.8⩽T⩽200\mathrm{K}$.

Figure 4

Mass magnetization of the foam as a function of the applied magnetic field, $M\left(H\right)$, at several temperatures from 1.8 to $92\mathrm{K}$. All data are corrected for the diamagnetic contribution of the gelatine sample holder. Inset: $M\left(H\right)$, hysteresis loop at $T=1.8\mathrm{K}$ exhibiting a coercive force $H_c=420\mathrm{Oe}$.

Figure 5

Corrected mass magnetization of the foam as a function of $H∕T$ measured at $T=1.8\mathrm{K}$. Solid line is a fit of the data to the Brillouin function with $S=1∕2$.

Figure 6

Magnetic moment as a function of the applied magnetic field for the gelatin sample holder measured at $T=5\mathrm{K}$ (crosses), and the composite of carbon nanofoam and sample holder measured at $T=5\mathrm{K}$ (closed circles) and $T=110\mathrm{K}$ (open circles).

Figure 7

Corrected mass magnetization as a function of $H∕T$ at $T=1.8\mathrm{K}$ (closed circles), $3\mathrm{K}$ (open circles), and $5\mathrm{K}$ (triangles). Note that the curves do not scale as it is expected for a PM and have the opposite behavior from that expected for a FM.

Figure 8

Temperature dependence of the magnetic moment of the composite of carbon nanofoam and sample holder measured at $H=30\mathrm{KOe}$, 15 days (upper curve) and 60 days (lower curve) after production of the sample.